A large number of techniques for purifying an exhaust gas generated from internal combustions have been conventionally proposed. In particular, various techniques for purifying an exhaust gas from diesel engine have been proposed for the purpose of reduction of discharge of particulate materials (PM) and NOx contained in an exhaust gas. For example, as catalysts for purifying an exhaust gas, oxidation catalysts which oxidize carbon monoxide (hereinafter also referred to as “CO”) and hydrocarbon (hereinafter also referred to as “HC”) into CO2 and H2O, NOx storage catalysts which store nitrogen oxides (hereinafter also referred to as “NOx”), three-way catalysts which simultaneously remove NOx, Colo. and HC, and the like have been proposed.
A catalyst for exhaust gas purification is required to have high thermal durability because it is exposed to exhaust gas at a high temperature. For example, Patent Literature 1 discloses fine mixed oxide powder obtained by uniformly dispersing Zr oxide and oxide of a metal M, which does not form a solid solution with the Zr oxide, in the nm scale. Patent Literature 1 discloses that the fine mixed oxide powder has a high specific surface area and a high pore volume, and therefore, when it supports a precious metal to form a catalyst, growth of particles of the precious metal is suppressed after aging at a high temperature.
Patent Literature 2 discloses an inorganic oxide powder containing secondary particles, which contain primary particles made of Al2O3, primary particles made of metal oxides of one or at least two of ZrO2, SiO2 and TiO2, and rare earth elements and/or rare earth oxides. The inorganic oxide powder is secondary particles obtained by dispersing primary particles made of Al2O3 and primary particles made of the above described metal oxides in a state of intervening each other and has thermal durability.